The present invention is directed to a system for detecting and measuring pressure distribution, and more particularly to a system which employs a pad to provide a quantitative indication of pressure distribution on a surface.
Pressure sensing pads are known for use in applications such as the detection of apnea or the like. They can be placed on the crib of an infant and are used to detect regular and periodic changes in pressure occasioned by the respiration and movement of the infant. If the infant should stop breathing, the failure to detect a change in pressure for a predetermined period of time provides an early indication of a possible alarm condition.
While most pressure sensing pads of the type known heretofore are capable of detecting the interruption of respiration, they are not suited for other uses. In particular, it is desirable to be able to measure the distribution of pressure on a surface. Information of this type would be invaluable to designers of sleep surfaces and furniture, for example. By knowing where the high-pressure points of a prone person are located, a bed can be designed which will more evenly distribute pressure and thereby diminish the occurrence of bedsores on patients who are confined for long periods of time. Similarly, the design of ergonomically efficient furniture is facilitated with such information.
Apnea monitoring pads are not suited for these types of applications because they are essentially qualitative measuring devices. In other words, they can detect if a pressure is being applied and whether it is changing, but they do not indicate how much pressure is being applied. Further they do not have the ability to provide any spatial resolution to the sensed pressure; they merely detect that it exists somewhere on the surface of the pad.
A significant problem that is encountered when attempts are made to provide spatial resolution of an applied pressure is the effect which various measuring points have on one another. For example, U.S. Pat. Nos. 4,134,063 and 4,437,138 disclose a pressure sensing pad that comprises a matrix of capacitive elements. Each capacitive element defines a measuring point. When a measurement is to be taken at a particular point, a voltage signal is applied to one terminal of the capacitor at that point, and a signal is obtained at the other terminal which is indicative of capacitance. Since the capacitance varies with the pressure on the pad at the location of the capacitor, the resulting signal provides pressure-related information.
However, since all of the capacitors are connected to each other in the matrix arrangement, errors can occur in the pressure measurement. More particularly, the signal obtained from one point will be influenced not only by the capacitance at that measuring point of interest but also by the capacitances of the surrounding points. Thus, changes in the surrounding pressures will be indicated in the measured signal and could result in erroneous readings.
The previously noted '063 patent contains a recognition of this problem. As a solution, it proposes that each of the input lines that supplies the voltage signal to the capacitors be connected to ground by a low-ohmic resistor. Apparently the resistor functions to shunt some of the error signal away from the output terminal of the capacitor of interest. While this approach attenuates the effect of the error on the measured signal, it would be preferable to eliminate or compensate the error signal to the greatest extent that can be practically obtained. For example, after supplying a voltage signal to one terminal of a selected capacitor of interest, capacitance is selectively measured at the other terminal of the capacitor by placing the latter terminal at an increased voltage potential. Such an increased voltage potential at the output terminal of the selected capacitor in the '063 patent is associated with the capacitor and diode utilized in the capacitance measuring voltage detection circuit GS.sub.k.
Accordingly, it is a general object of the present invention to provide a novel capacitive system for measuring the spatial distribution of pressure.
It is a more specific object along these lines to provide such a system which eliminates the errors occasioned by interaction among commonly connected capacitors which affect the measured signal.
It is a further object of the invention to also eliminate the effects of noise from other sources which can introduce error into the pressure measurement.
It is a further object of the invention to provide a pad structure which does not generate noise that could interfere with external instrumentation used in the environment of a patient.
It is a further object of the invention to provide a novel pad structure for providing capacitive type measurements.